The invention relates to a method for determining the spatial distribution of magnetic resonance signals from a predetermined imaging region that is completely covered by N MSEM regions (MSEM=monotonic spatially encoding magnetic field) within a volume under examination of a magnetic resonance apparatus, with N≧1, wherein, in a preparatory step, a spatial encoding scheme P with M encoding steps, M≧1, for spatial encoding in L spatial dimensions within the imaging region is defined; wherein, in an execution step, for each encoding step of the spatial encoding scheme P, nuclear spins are excited in the volume under examination by at least one RF pulse by means of RF transmitter antenna equipment having at least one transmitter element; after this RF excitation, spatial encoding is performed according to the spatial encoding scheme P by means of additional magnetic fields, variable over time and space, of a global and/or local gradient system, wherein the spatial encoding is performed in at least one spatial dimension by means of the local gradient system and is unique within each of the N MSEM regions, which are non-overlapping, but is not unique for multiple MSEM regions and not in the entire volume under examination, and wherein the spatial encoding performed by the global gradient system is unique in the entire volume under examination with respect to each of the dimensions to be mapped, and magnetic resonance signals generated by the excited nuclear spins are acquired by means of RF receiver antenna equipment with at least one receiver element; wherein, in a reconstruction step, one or more spatial distributions of the magnetic resonance signals or quantities derived from them are calculated from the magnetic resonance signals acquired in all encoding steps according to the spatial encoding scheme P; and wherein, in a visualization step, the results of the reconstruction and/or one or more quantities derived from them are stored and/or visualized.
Such methods are known from [1], [2], and [3]. In these methods known from prior art, the unique determination of the spatial distribution of the magnetic resonance signals is performed for the entire imaging region in all dimensions to be mapped by means of RF receiver equipment with at least N elements, where N is the number of regions with locally uniquely spatially encoding additional magnetic fields and each receiver element exhibits different sensitivities for magnetic resonance signals from different regions. According to [1] and [2], one RF receiver element is deployed for each of these regions, whose receiver sensitivity spatially focuses on this region.
Magnetic resonance imaging (MRI), also termed magnetic resonance tomography (MRT), and spatially resolved magnetic resonance spectroscopy (MRS), also termed spectroscopic imaging (SI), chemical shift imaging (CSI) or multi-voxel localization MRS, are widespread techniques for non-destructive acquisition of images of the interior of an object under examination and are based on the spatially resolved measurement of magnetic resonance signals from the object under examination. By subjecting the object under examination to an essentially static and homogenous magnetic basic field inside a basic field magnet, nuclear spins contained in it are oriented in the direction of the basic field, usually selected as the z-direction of a magnet-bound coordinate system. During an MR examination, irradiation with electromagnetic radio-frequency (RF) pulses by means of one or more RF transmission antennas excites the such oriented nuclear spins of the object under examination to precession movements whose frequencies are proportional to the local magnetic field strengths. In the case of the MRI and SI methods generally used today, spatial encoding is imposed on the precession movements of the nuclear spins by superpositions varying over time of additional magnetic fields varying over space, for all spatially resolving spatial directions. These fields are usually produced in three variants as magnetic fields increasing linearly over space in three orthogonal spatial directions by means of a gradient system and termed gradient (fields) Gx, Gy, and Gz. The spatial encoding is usually described by a scheme in so-called k-space, which is related to geometric or physical space via a Fourier transformation. The transverse component of the magnetization associated with the processing nuclear spins induces electrical voltage signals in one or more RF receiver antennas, which usually surround the object under examination. By means of pulse sequences that contain specially selected trains of RF pulses and gradient pulses, magnetic resonance signals that are variable over time are generated in such a way that they can be converted to corresponding spatial images. This is performed according to one of many well-known reconstruction techniques, after the RF signals have been acquired, amplified, and digitized using an electronic receiver system, and processed and stored in two- or multidimensional datasets using a computer system. The pulse sequence used typically contains a succession of measurement procedures, also termed spatial encoding steps, in which the gradient pulses vary according to the selected localization method in accordance with the encoding scheme used. A single spatial encoding step comprises the excitation of nuclear spins, spatial encoding, and the acquisition of MR signals.
One essential requirement for accurate spatial mapping of the magnetic resonance signals of the object under examination is that the technical imperfections of the MR measurement system are negligible or the deviations from the ideal behavior are known and can be corrected accordingly.
In magnetic resonance imaging and spatially resolved magnetic resonance spectroscopy, spatial localization is usually achieved by performing either Fourier encoding or spatially selective excitation [7, 8].
In Fourier encoding, the nuclear spins to be examined are excited simultaneously in the entire volume under examination and their spatial localization is implemented by superposing a spatially dependent phase and/or frequency encoding on their precession movements. This superposition of the spatial encoding is performed using gradient pulses, that is, variations over time of the magnetic field strength of one or more variants of the additional magnetic fields produced by the gradient system. In classic MR imaging, this superposition is performed on the one hand by applying a so-called phase (encoding) gradient in a phase encoding period following RF excitation, in which spatially dependent phase modification of the precession movement occurs, on the other hand by application of a readout gradient during signal readout, resulting in spatially dependent modulation of the precession frequency. Both encodings are usually performed according to an encoding scheme that permits determination of the spatial distribution of the magnetic resonance signals by means of a Fourier transformation.
In backprojection imaging, the spatial encoding in two or three dimensions is performed solely by frequency encoding by variation of the orientation of the readout gradient in each encoding step.
The spatially selective excitation is a widespread technique in magnetic resonance imaging, which is used to spatially limit the transverse magnetization produced during excitation and/or to spatially vary its amplitude and phase in the excitation volume. In slice selection, which is the most frequent case of selective excitation, the excitation volume is reduced to a defined slice. Also in volume-selective MR spectroscopy (VSS), selection of a region under examination—usually small in relation to the object under examination—is usually based on slice-selective excitation and refocusing pulses, with the spatial selection being performed successively in one spatial direction at a time by means of a corresponding gradient pulse.
To speed up multiple slice acquisitions, MRI and MRS methods have also been developed in which, in multiple phase encoding steps, multiple essentially parallel slices are simultaneously excited with different phase encoding, their magnetic resonance signals are acquired, and the signals are assigned to the relevant excitation slice by means of suitable data reconstruction, e.g. a Hadamard transformation [9].
The multidimensional selective excitation by means of multidimensional RF pulses [10, 11, 12], in which the excitation volume is limited in more than one direction or the excitation is modulated in more than one direction, has also given rise to numerous applications. Noteworthy examples in this case include the excitation of a small three-dimensional volume or multiple volumes simultaneously within a much larger object under examination for localized spectroscopy, the imaging of a selectively excited region of interest (ROI) with a reduced field of view (FOV) for the purpose of reducing the measurement time, the excitation of special volumes adapted to the structures of the object under examination, and echo-planar imaging with reduced echo train lengths. The amplitude and phase modulation in the excitation can also be used to compensate for disadvantageous effects of an inhomogeneous B1 field of the RF antennas used for transmission. This is an application that has become immensely more important because of the large increase in high-field MRI systems [12, 13].
MRI and MRS methods are also known in which nuclear spins are simultaneously selectively excited within one or more spatially separated regions under examination by means of multidimensional RF excitation, and phase encoding is superposed on the magnetic resonance signals during this excitation by means of a suitable encoding scheme. In simultaneous acquisition of the magnetic resonance signals of all regions under examination, this phase encoding permits separation of the signals based on their region of origin and/or determination of their spatial distribution within these regions [14, 15, 16].
For the practical deployment of multidimensional RF pulses, a further aspect of technical progress over the past few years has proven advantageous and is described in detail in [12]. In the past, spatially selective excitation has usually been performed using a single RF transmitter antenna, having an essentially homogeneous transmission field (B1 field), in conjunction with the gradient system. Inspired by the success of parallel imaging, in which signal acquisition is performed with a configuration of multiple RF receiver antennas, also termed antenna array in the specialist literature, one has passed on to now use such arrays for transmission in selective excitation as well. This makes it possible to partially replace the spatial encoding of the excitation locations, which is implemented in selective excitation by variation of gradient fields by analogy with acquisition, by so-called sensitivity encoding and thereby to reduce the length of the excitation pulses. This means that information is used that is contained in the different spatial variations of the transmission fields of the individual array elements, hereinafter also referred to as transmission profiles [17, 18].
One of the basic issues in the use of spatially selective excitation is determination of the RF pulses that have to be played out by the transmitter antenna equipment to generate the desired excitation pattern in conjunction with the k-space trajectory produced with the gradients. In [10], Pauly et al. describe a method for one-channel spatially selective excitation in which, because of a mathematical analogy of selective excitation with Fourier imaging, the sought pulse shape B1(t) can be essentially calculated by Fourier transformation of the desired excitation pattern and sampling of the Fourier transformation along the defined k-space trajectory. Katscher et al. extended this calculation method to an antenna array having multiple independent transmission channels [17].
In addition to these methods of selective excitation that are characterized by the fact that, during excitation of the nuclear spins by RF pulses, gradient pulses with a spatially encoding effect are simultaneously applied, techniques have also been developed in which, without the additional effect of gradient fields, spatial amplitude and/or phase modulation of the transverse magnetization is achieved by pure superposition of accordingly designed RF pulses that are irradiated simultaneously using at least two transmitter antenna elements [19, 20, 21].
In magnetic resonance imaging and in spatially resolved magnetic resonance spectroscopy, gradient fields that rise or fall monotonically in one spatial direction in the entire volume under examination of the nuclear resonance apparatus are usually used for spatial encoding of the magnetic resonance signals. Because of this property of covering the entire volume under examination, these gradient fields are termed global gradients and the generating system component is termed a global gradient system. Moreover, to simplify representation in the following description, it is assumed that the basic field is oriented in the z-direction of a magnet-bound coordinate system and that the gradient fields can be switched in three variants Ggx, Ggy, and Ggz, whose z-components essentially increase linearly in mutually orthogonal directions with a settable strength [7, 8].
The application of the strongest possible gradients, that is, the formation of the greatest possible magnetic field difference between the edges of the imaging region provides considerable advantages including the implementation of very high spatial resolution. The fastest possible switching response when these gradients are switched on and off and when the gradient strength is set is also advantageous, for example, to shorten the total measurement process.
It is a disadvantage in the deployment of global gradients that the gradient strengths required for typical applications correspond to considerable magnetic field differences between the edges of the imaging region. Their implementation reaches the technical limits of gradient coil design and of dimensioning of the gradient amplifiers with respect to the magnitude and the switching response of the electrical currents to be generated by the gradients coils. Moreover, quickly varying Lorentz forces occur during fast switching of these magnetic fields that can disadvantageously result in very large mechanical stresses on the nuclear resonance apparatus and excessive noise production. A further limitation are neural stimulations in living objects under examination by rapidly varying large magnetic field strengths so that in many cases the image quality that could be technically achieved in principle cannot be implemented in practice because of physiological restrictions re acoustical strain or nerve stimulation.
To avoid these limitations of global gradients, so-called local gradients are introduced. These are usually produced by a local gradient system included in the apparatus in addition to the global gradient system. With such a local gradient system, additional magnetic fields are produced to be used for spatial encoding, wherein for each spatial dimension to be encoded a corresponding variant of the additional magnetic field is implemented with a different respective local gradient                (∂Bz(x,y,z)/∂x, ∂Bz(x,y,z)/∂y, ∂Bz(x,y,z)/∂z)of its z-component Bz(x,y,z). As in the case of global gradients, with local gradients each of these variants of the spatially encoding additional magnetic field usually has the property that its z-component Bz(x,y,z) is homogeneously scalable up to a maximum strength in the entire volume under examination. Unlike global gradients, the z-component Bz(x,y,z) of each individual variant of the additional magnetic field of local gradients rises or falls monotonically along the field lines of its gradient field (∂Bz(x,y,z)/∂x, ∂Bz(x,y,z)/∂y, ∂Bz(x,y,z)/∂z) within only one or several extended and connected subregions of the volume under examination. There is no continuous monotonic progression along these field lines throughout the entire volume under examination. With a variant of such a local additional magnetic field, one-dimensional spatial determination can be performed along the field lines, and components of the acquired magnetic resonance signals can be assigned to individual isosurfaces of the z-component Bz(x,y,z) of the corresponding variant of the additional magnetic field. These magnetic field isosurfaces are locally perpendicular to the field lines and must be known for spatial reconstruction from the magnetic resonance signals. If spatial determination of the magnetic resonance signals is to be performed in multiple spatial dimensions, a corresponding number of suitable variants of the additional magnetic field, hereinafter termed Gl1, Gl2, . . . , are required and can also be active simultaneously depending on the encoding method. In the case of multidimensional spatial encoding by means of the local gradient system, unique spatial encoding can only be performed in the regions in which all variants of the additional magnetic field therefor used have a monotonic progression, as described above. These regions in which unique spatial encoding can be performed for all the desired dimensions, are hereinafter called MSEM regions.        
The advantage obtained from such local gradients is that, within each of these MSEM regions, a spatially encoding magnetic field is produced with a very steep rise and rapid switching response, which can be used to increase the spatial resolution and/or shorten the measurement process. Because the magnetic field difference between the edges of an MSEM region and therefore also the magnetic field variation within the entire volume under examination can be kept much smaller than in the case of global gradients, the disadvantages stated above of large magnetic field variations over time within the nuclear resonance apparatus are greatly reduced or avoided.
If magnetic resonance signals from the entire volume under examination are spatially encoded using such a local gradient system, globally unique spatial assignment based on this spatial encoding is generally not possible in the event of non-spatially selective signal generation or acquisition. In the case of a single MSEM region, it cannot be ascertained which portion of the signal originates from that region and which portion from outside of the region; in the case of multiple MSEM regions, it is furthermore generally not possible to distinguish which signal components originate from which of these MSEM regions. In the case of a single MSEM region, this problem is usually solved by deploying excitation and/or receiver antennas with limited spatial sensitivity, in particular, surface coils, so that only magnetic resonance signals within this MSEM region are excited and/or measured. In the case of local gradient systems that have multiple MSEM regions, unique assignment of the signals is achieved by deploying an antenna array with at least as many suitably disposed elements of different sensitivity, enabling unique assignment as described in [1] to [6]. This may require a complex reconstruction method, for example, a SENSE-like reconstruction [3] to [6].
The disadvantage of this prior art is firstly the necessity to use RF coils or coil arrays with limited sensitivity profiles, given that with the number of MSEM regions the number of receiver elements and channels increases accordingly, making the apparatus much more complex. Moreover, the image reconstruction method is very complex and can result in image artifacts under non-ideal measuring conditions. Because the coils used have especially high sensitivity for object regions near to the surface and much reduced sensitivity for low-lying regions, the methods of prior art are unsuitable for selection of MSEM regions that are removed from the surface.
Using these methods in conjunction with techniques of parallel imaging [7] results in the disadvantage that the potential of the multi-element receiver coils to shorten the measurement time can only be exploited to a limited degree because part of the additional measurement information first has to be used to identify the MSEM region producing the signal. This disadvantage can only be compensated for by a corresponding increase in the number of receiver elements and channels, that is, by more complex apparatus.
It is therefore the object of the invention to provide a measurement method and a reconstruction method that, using local gradient systems with a much less complex RF reception configuration, permit unique determination of the spatial distribution of the magnetic resonance signals in the entire imaging region, in particular, that are also suitable for the imaging of MSEM regions that are remote from the surface, and can effectively be deployed in conjunction with parallel imaging techniques.